1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine having a first fuel injection mechanism (an in-cylinder injector) for injecting a fuel into a cylinder and a second fuel injection mechanism (an intake manifold injector) for injecting a fuel into an intake manifold or an intake port, and relates particularly to a technique for determining a fuel injection ratio between the first and second fuel injection mechanisms.
2. Description of the Background Art
An internal combustion engine having a first fuel injection valve (an intake manifold injector in the background art) for injecting a fuel into an intake manifold of the engine and a second fuel injection valve (an in-cylinder injector in the background art) for always injecting a fuel into a combustion chamber of the engine, and configured to stop fuel injection from the first fuel injection valve (the intake manifold injector) when the engine load is lower than a preset load and to cause fuel injection from the first fuel injection valve (the intake manifold injector) when the engine load is higher than the set load, is known.
In such an internal combustion engine, one configured to switch between stratified charge combustion and homogeneous combustion in accordance with its operation state is known. In the stratified charge combustion, the fuel is injected from the in-cylinder injector during a compression stroke to form a stratified air-fuel mixture locally around a spark plug, for lean combustion of the fuel. In the homogeneous combustion, the fuel is diffused in the combustion chamber to form a homogeneous air-fuel mixture, for combustion of the fuel.
Japanese Patent Laying-Open No. 2001-020837 discloses a fuel injection control apparatus for an engine that switches between stratified charge combustion and homogeneous combustion in accordance with an operation state and that has a main fuel injection valve for injecting a fuel directly into a combustion chamber and a secondary fuel injection valve for injecting a fuel into an intake port of each cylinder. This fuel injection control apparatus for the engine is characterized in that the fuel injection ratio between the main fuel injection valve and the secondary fuel injection valve is set in a variable manner based on an operation state of the engine.
According to this fuel injection control apparatus for the engine, the stratified charge combustion is carried out using only the main fuel injection valve directly injecting the fuel into the combustion chamber, while the homogeneous combustion is carried out using both the main fuel injection valve and the secondary fuel injection valve (or using only the secondary fuel injection valve in some cases). This can keep the capacity of the main fuel injection valve small, even in the case of an engine of high power. Linearity in injection duration/injection quantity characteristic of the main fuel injection valve in a low-load region such as during idling is improved, which in turn improves accuracy in control of the fuel injection quantity. Accordingly, it is possible to maintain favorable stratified charge combustion, and thus to improve stability of the low-load operation such as idling. In the homogeneous combustion, both the main and secondary fuel injection valves are employed, so that the benefit of the direct fuel injection and the benefit of the intake port injection are both enjoyed. Accordingly, favorable homogeneous combustion can also be maintained.
In the fuel injection control apparatus for the engine disclosed in Japanese Patent Laying-Open No. 2001-020837, the stratified charge combustion and the homogeneous combustion are employed according to the situations, which complicates ignition control, injection control and throttle control, and requires control programs corresponding to the respective combustion manners. Particularly, upon switching between the combustion manners, these controls require considerable changes, making it difficult to realize desirable controls (of fuel efficiency, emission purification performance) at the time of transition. Further, in the stratified combustion region where lean combustion is carried out, the three-way catalyst does not work, in which case a lean NOx catalyst needs to be used, leading to an increased cost.
Based on the foregoing, a direct injection engine has been developed which has only an in-cylinder injector to carry out homogeneous combustion over the entire region, with no stratified charge combustion conducted, and thus does not need control for switching between the stratified charge combustion and the homogeneous combustion and does not require an expensive lean NOx catalyst.
In such a direct injection engine, however, the homogeneous combustion is carried out over the entire region using only the in-cylinder injector. This may lead to insufficient homogeneity and large torque fluctuations in the low-speed and high-load state of the engine. Japanese Patent Laying-Open No. 2001-020837 described above merely discloses that in the region where homogeneous combustion is carried out, a ratio of the quantity of the fuel injected from the secondary fuel injection valve injecting the fuel into the intake port with respect to the total quantity of the fuel injected is increased in accordance with an increase of the engine output (engine speed and load), which cannot provide solutions to the above-described problems.